Method of producing a thin steel sheet having baking hardenability and adapted for drawing

Abstract

The present invention provides a method for producing a thin cold rolled steel sheet, which is adapted to be formed into, for example, an external automotive plate. The method includes an appearance-finishing step by bake-coating as a final step after drawing (such as press molding), and has a remarkably high baking hardenability. The baking hardenability is a property for improving the yield strength due to the heating during the bake coating treatment and is particularly advantageous for producing a light weight automobile and for compensating the deterioration of dent resistance of the automotive plate due to the lowering of its weight, (without deteriorating the r value). The latter is an indication of the press moldability of the thin steel sheet. Furthermore, the present invention discloses an effective compounding amount of Ti which acts to fix C, S and N contained in the steel, and a continuous annealing condition properly selected depending upon the effective amount of Ti.

We claim: 
     
       1. A method of producing a thin steel sheet adapted for drawing, comprising subjecting a cold rolled thin steel, which has a composition containing 0.001-0.010% by weight of C, not more than 1.0% by weight of Mn, not more than 1.2% by weight of Si, not more than 0.1% by weight of P, not more than 0.02% by weight of S and not more than 0.01% by weight of N, and further containing effective Ti in an amount larger than (4 times amount of [C] content minus 0.015% by weight ) but smaller than (4 times amount of [C] content plus 0.05% by weight) said effective Ti being remaining Ti after (48/32[S]+48/14[N])% by weight of Ti is substracted from the total amount of Ti depending upon the [S] and [N] contents in the steel, to a continuous annealing under a condition that the sheet is heated at a temperature, which lies within the range of from 850° to 950° C. and further lies within the range of from 850° C.+(70/0.05){[effective Ti]% by weight-4% by weight ° C. to 950° C.+(100/0.015){[effective Ti] % by weight -4[C]% by weight}° C. depending upon the amount of [effective Ti] and [C] content, for a period of from 10 seconds to 5 minutes, and the above heated sheet is rapidly cooled at a cooling rate of at least 10° C. per second within the temperature range of 850°-500° C. during the cooling step following to the heating, so as to obtain the thin steel sheet having a striking energy-absorbing property of at least 6 kgm/cm 2  in a Charpy impact test at -100° C., a r value of at least 1.8 and a baking hardenability of at least 4 kg/mm 2 .